In recent years, electronic equipment has attained lower power consumption, greater functionality, and higher speed. With this trend, semiconductor devices associated with the equipment have also been required to attain lower power consumption and higher speed. To meet this requirement, semiconductor devices generally used for load switches and DC-DC converters of electronic equipment are required to have transistors with smaller on-resistances.
One approach to decreasing the on-resistance of a transistor is that a device is miniaturized to increase the density of transistors arranged per unit area. Examples of this approach include a method for fabricating a vertical gate semiconductor device in which a trench is formed in a substrate and a gate insulating film and a gate electrode are formed inside the trench. The vertical gate semiconductor device can have an increased transistor density by arranging the trenches in a stripe pattern and making the width of each trench finer and also the pitch between the adjacent trenches smaller.
In the vertical gate semiconductor device, a gate lead portion (a gate connection portion) is provided in order to lead the gate electrode to the outside of the trench to bring the electrode into electric contact with an aluminum interconnect or the like. FIG. 12 is a plan view showing the structure of a conventional vertical gate semiconductor device. Referring to FIG. 12, in the conventional vertical gate semiconductor device, the semiconductor substrate 100 is provided with a plurality of trenches 101, and each of the trenches 101 is provided with a gate electrode portion 102 and a gate lead portion 103.
For the vertical gate semiconductor device thus constructed, thermal oxidation is typically conducted to form a gate insulating film on the inner wall of the trench. This thermal oxidation process, however, causes dislocations or defects significantly in a discontinuous portion of the trench, particularly in the gate lead portion located at the end of the trench. To solve such a problem, another approach is proposed in which a plurality of gate electrodes are connected to each other by their respective gate lead portions with gently varying curvatures (see Patent Document 1).
This approach will now be described. FIGS. 13(a) and 13(b) are a plan view and a sectional view showing the structure of a conventional vertical gate semiconductor device, respectively. Referring to FIG. 13(a), in the conventional vertical gate semiconductor device, an end 112 of a trench 111 is provided in a gate lead portion 113. The end 112 bifurcates, and the respective bifurcated portions are connected to ends 112 of adjacent trenches 111. As shown in FIG. 13(b), in the cross section of the end 112, an n−-epitaxial layer 122 and a p-type base diffusion layer 123 are formed on an n+-semiconductor substrate 121. The surface of the end of the trench 111 is formed with a gate insulating film 124 and a polysilicon film 125.
The structure as shown above is employed to eliminate the discontinuous portion of the trench. Therefore, in the thermal oxidation process, stress induced by the oxidation can be reduced.
Patent Document 1: Japanese Patent No. 3367857